Thermoplastic resins which have hitherto been known as low-modulus resins and resins which are soluble in solvents have a glass transition point below or around room temperature and have had a problem concerning high-temperature reliability. For example, in applications such as the mounting of semiconductor parts on a substrate, a temperature of 250° C. or higher is practically necessary for solder melting. However, the resins having a low glass transition point melt at such temperatures, and the resins themselves deform and are unsuitable for such use. Furthermore, there have been problems concerning the formation of voids or interfacial separation attributable to the volatilization or condensation of water contained in these resins, and concerning resultant package cracking, etc.
For eliminating those problems, investigations were made on a curable resin having a crosslinking structure introduced therein or on a resin having a high glass transition temperature. However, the curable resin has a problem that stress generation by cure shrinkage is unavoidable.
On the other hand, the resin having a high glass transition point has a high modulus of elasticity at desired heat-resistance temperature. Because of this high modulus of elasticity, the stress generated by a difference in the coefficient of thermal (linear) expansion between the substrate and the resin during a high-temperature processing or subsequent cooling may pose a problem. In this case, no problem arises when the coefficient of thermal (linear) expansion of each of the materials constituting the structure to be formed is equal to the coefficient of thermal (linear) expansion of the resin in the temperature range in each processing. However, such structures have had a problem that the design itself of the structures is considerably restricted.
Low-modulus polyimides are being used as a low-stress heat-resistant material for forming protective films for semiconductors, insulating films in multilayered circuit boards, adhesive films for semiconductors, coverlays for flexible circuit boards, and the like (see patent documents 1 to 6).
However, these low-modulus polyimides have been unable to be used in applications where contamination by a silicone may arouse a serious trouble, such as HDD applications and some semiconductor applications, since these low-modulus polyimides are obtained by copolymerizing a diamine or tetracarboxylic anhydride containing a silicone.
There has hence been a desire for a low-modulus heat-resistant resin which can be used in, e.g., an apparatus for HDD or semiconductor production without causing contamination.    Patent Document 1: JP-A-5-170901    Patent Document 2: JP-A-6-73178    Patent Document 3: JP-A-6-207024    Patent Document 4: JP-A-6-73178    Patent Document 5: JP-A-2002-50854    Patent Document 6: JP-A-2000-103848